Posted: Mon Jan 01, 2018 2:34 am
by jcruddat
Hi all,
I have recently taken on the challenge of machining a 28" to 30" long increment borer at the CNC machine shop of my school, Worcester Polytechnic Institute, for the purpose of drilling closer to the pith of the 150+ year old tulip poplars, 250+ year old sugar maples, and other old growth trees here in Connecticut. So far I have made a fair virtual 3d representation of the threaded tip in a CAD program called SolidWorks which is available to view or download at I you wish to view the file but do not have access to SolidWorks then I can upload the file here in a different format such as in the .STL or .OBJ file format, viewable in free software. Right now I am trying to refine the design of the threads since the rest of the borer is pretty straight forward. I am currently trying to base the threads on the JIM-GEM style borer which looks very similar to the old Djos/Suunto style borer. I prefer the JIM-GEM borer over the more common Haglöf borer because the spreader bars are made in-line with threads instead of the behind the threads. This allows for much easier backing out of the borer from the tree since there are still threads behind the cams re-engaging with the sometimes soft or rotten wood. The previously compressed walls of the wood surrounding the hole from the spreader bars tends to spring back into place and can sometimes lock the Haglöf borer in place. I have had the Haglöf borers get stuck multiple times, usually when hitting a pocket of rot or cavity within the tree, but almost never with the JIM-GEM borer. The hardest part of modeling the threads is the fact that the geometry of the threaded borer tip is very difficult to measure such as the curvature of the face of the cone/barrel that the threads are made on, along with the geometry spreader bars that are in-line with the threads. Does anyone have information as to the dimensions of the tip of the borer or the process in which they are machined? Also, if anyone has some input as to the most effective style of increment borer then that would also be very helpful. So far I have not been able to find much useful information on the internet as to how they are machined or how they are dimensioned other than one simple schematic and a cross section view both attached at the end of this post. I assume that the threads are machine on a 5-axis CNC mill or lathe and then welded to the shaft along with the square end that fits into the handle. I have heard that the old Djos, Mattson, and particularly the Sandvik increment borers were of well respected and rarely broke or got stuck. The Djos style borer looks similar to the JIM-GEM style borer I am using now but I have not been able to find any information or photos on the geometry of the old Sandvik borers. Perhaps someone has one lying around and could post a picture of the threads or some insight as to the schematics of such devices?

Jack Ruddat